Protein kinases regulate various functions in the cell including cell growth, proliferation and survival. Dysregulation of protein kinases is often the cause of many solid malignancies (Manning G. et al. Science. 2002, 298, 1912-1934). The use of protein kinase inhibitors has led to substantial clinical benefit in patients harboring oncogenic aberrations. More than thirty protein kinase inhibitors have been approved for clinical treatment of cancer (Berndt N. et al. Curr. Opin. Chem. Biol. 2017, 39:126-132). RET is a receptor tyrosine kinase that was initially discovered in 1985 through transfection of NIH3T3 cells with human lymphoma DNA (Takahashi, M. et al. Cell. 1985, 42:581-588.). RET is expressed with its highest levels in early embryogenesis (during which it has diverse roles in different tissues) and decreases to relatively low levels in normal adult tissues Pachnis, V., et al. Development 1993, 119, 1005-1017). RET plays a critical role in the development of enteric nervous system and kidneys during embryogenesis (Schuchardt, A. et al. Nature 1994, 367:380-383). RET activation regulates the downstream signalling pathways (RAS/MAPK/ERK, PI3K/AKT, and JAK-STAT etc.), leading to cellular proliferation, migration, and differentiation (Mulligan, L M. Nat Rev Cancer. 2014, 14(3):173-86).
Gain-of-function mutations of RET with constitutive activation have been found in heritable and sporadic tumors including activating point mutations within the full-length RET protein or genomic rearrangements that produce chimeric RET oncoproteins in the cytosol. The heritable oncogenic RET mutations are found in multiple endocrine neoplasia type 2 (MEN2) including medullary thyroid cancer (MTC) and familial MTC with more than 80 pathogenic variants spanning RET exons 5-16 reported (Mulligan, L M. Nat Rev Cancer. 2014, 14(3):173-86). Among them, RET M918T and RET A883F are found in 40-65% of sporadic MTC. The somatic mutation, chimeric RET fusion oncoproteins have been identified in sporadic tumors. The RET rearrangements are originally reported in papillary thyroid cancers (PTCs) (Grieco, M. et al. Cell. 1990, 23; 60 (4):557-63.). The resulting fusion transcripts composed of the 3′ end of RET kinase domain and the 5′ end of separate partner genes (CCDC6, NCOA4, TRIM24, TRIM33, PRKAR1A, GOLGA5, KTN1, ERC1, MBD1, and TRIM27 etc.). RET fusions are identified in approximately 20%-40% of PTCs, and CCDC6-RET and NCOA4-RET are the most commonly identified RET fusions in PTCs (Drilon A, et al. Nat Rev Clin Oncol. 2017 Nov. 14. doi: 10.1038/nrclinonc.2017.175). RET gene fusions are also found in approximately 1%-2% of non-small cell lung cancer (NSCLC) (Gainor J F, Shaw A T. Oncologist. 2013, 18(7):865-75), and over 50% of RET fusions in NSCLC is KIF5B-RET, representing the most frequent RET fusion form. However, the RET inhibitors have relatively low response rates and short treatment duration in the treatment of NSCLC patients with KIF5B-RET fusion gene in multiple clinical trials (Drilon, A. Nat Rev Clin Oncol. 2017 Nov. 14. doi: 10.1038/nrclinonc. 2017.175). It was reported that the kinesin and kinase domains of KIF5B-RET act together to establish an emergent microtubule and RAB-vesicle-dependent RET-SRC-EGFR-FGFR signaling hub (Das T K and Cagan R L Cell Rep. 2017, 20(10):2368-2383). The inhibition of SRC kinase will have the potential to stop the recruitment of multiple RTKs via the N terminus of the KIF5B-RET fusion protein and the oncogenic signaling to increase the therapeutic efficiency of RET inhibitors. In addition, Src family tyrosine kinases regulate MTC cellular proliferation in vitro and mediate growth signals by increasing DNA synthesis and decreasing apoptosis (Liu Z, et al. J. Clin. Endocrinol. Metab. 2004, 89, 3503-3509). Therefore, a dual inhibitor of RET and SRC represents a highly desired therapeutic intervention to maximally target abnormal RET signaling in cancers.